1. Field of the Invention
The present invention relates to an image processing system for processing digital image data and its image smoothing method, and more particularly, to an image processing system and its smoothing method characterized by the means employed for correcting an input image recorded in a color fog state and in a backlight state.
2. Description of the Related Art
When processing digital image data by use of a computer-based image processing system, various forms of smoothing are performed on the image data as necessary. Hereafter, color fog and backlight correction by use of a conventional image processing systems is described.
First, color fog correction is described. In the color fog correcting method used by conventional image processing systems, the tone curve of the RGB is interactively modified; the operator corrects the colors of an image while confirming the results of his operation.
This conventional method focusing on smoothing the tone curve of the RGB interactively, however, requires too many adjustment parameters for the operator to perform smoothing intuitively, thereby giving rise to a drawback of longer operation time. Furthermore, this method requires experience and skill so that those having no or little experience are unable to perform smoothing as desired.
This type of conventional image processing system for smoothing color is disclosed, for example, in Japanese Patent Laid-Open Publication No. 2-94893, "Image Processing System". The same publication describes a system which allows correction depending on the spectral properties of different types of light source.
The conventional image processing system disclosed in the same publication is described with reference to FIG. 14. In FIG. 14, a spectral properties storing means 1402 stores the amounts of correction to be made for three color components, i.e., red, green, and blue, depending on the spectral properties of different types of light source. A correction amount retrieving means 1403 retrieves the amount of correction for a specified light source from the spectral properties storing means 1402 and inputs it into a color correcting means 1401. The data stored in an input image buffer 1400 is subjected to color correction by the color correcting means 1401 and output to an output image buffer 1404. Assuming that the input pixel values are R, G, and B, and that the output pixel values are R', G', and B', color correction is performed using the following expression (1). EQU R'=R/r0, G'=G/g0, B'=B/b0 (1)
This correcting method multiplies the RGB value by the constant term.
In the correcting method that simply multiplies the RGB values by the constant term, however, it is impossible to smooth all the colors of respective light sources completely, resulting in the inability to achieve the best possible color through correction. In addition, a color fog state that has been caused by such factors as film, lens, and exposure characteristics, along with color variances among different light sources, is impossible to correct solely based on the properties of different light sources, making this method inadequate for obtaining good correction results.
Next, backlight correction is described. Conventional backlight correcting systems include an automatic exposure control device, for example, a video camera. This type of system controls the diaphragm so as to maintain the level of output signals constant. Means of controlling the diaphragm include the mean value method that obtains the mean value of brightness of an entire screen, the peak value method that detects the maximum value of brightness of a screen, and the method that combines both. A diaphragm control unit, typically located behind a lens system, controls the quantity of incident light to make the screen brighter or darker. This type of conventional backlight correcting system is disclosed, for example, in Japanese Patent Laid-Open Publication No. 6-46325, "Automatic Exposure Control System". The same publication describes a system that estimates the intensity of backlight based on the brightness data for a screen and determines how much the diaphragm should be controlled using the value thus obtained.
The conventional backlight correction device disclosed in the same publication is described with reference to FIG. 15. In FIG. 15, a subject image is formed on an image pickup device 1503 via a lens 1501 and a diaphragm 1502, converted into an electric signal, and output through a signal processing circuit 1505 for performing the .gamma. processing and other tasks. At this time, the diaphragm control is performed by use of a signal from the image pickup device 1503 as follows.
A full screen mean value detecting unit 1507 detects the mean brightness of the full screen. Subsequently, the diaphragm control unit 1506 determines the diaphragm aperture so as to achieve a target brightness. A smaller region mean value detecting unit 1508 divides a screen into smaller regions and calculates the mean value of each block. A low brightness mean value calculating unit 1509 arranges in the order of brightness the mean values of brightness of respective small regions calculated by the smaller region mean value detecting unit 1508, and selects a predetermined number of small regions sequentially from the small region of the lowest brightness, and calculates the mean brightness of the regions thus selected. A backlight degree calculating unit 1511 calculates the degree of backlight from the mean brightness of the several selected regions calculated by the low brightness mean value calculating unit 1509. A target brightness calculating unit 1510 modifies the target aperture so as to enlarge the diaphragm aperture when the degree of the backlight calculated by the backlight degree calculating unit 1511 becomes larger, and modifies the target aperture so as to reduce the diaphragm aperture when the degree of the backlight becomes smaller.
In conventional backlight correcting systems, backlight is corrected by adjusting the diaphragm aperture so as to enter the greater quantity of light or the smaller quantity of light. This aims to control the device, a video camera or the like, directly. The similar correcting method cannot be used for the data once quantized and stored as a digital image.
Considering diaphragm control as image processing, .gamma. correction conversion as illustrated in FIG. 16 is the closest conversion to backlight correction through the above-mentioned modification of the diaphragm aperture. When diaphragm is not changed, the input value of brightness is the same as the output value of brightness, as illustrated in the conversion characteristic 1603. An enlarged diaphragm aperture has characteristic as illustrated in the conversion characteristic 1601 for a large aperture and a reduced diaphragm aperture has the characteristic as illustrated in the conversion characteristic 1602 for a small aperture.
Since the range of partial light quantities in the scenery of the natural environment is being recorded by a video camera, the brightness value in the shadow region (the darkest region in the image) will not be increased uniformly when the diaphragm is enlarged; instead, more information on the shadow region is recorded as contrast intensifies. In the case of image data, however, since brightness information on the shadow region has already been quantized and compressed, .gamma. correction would not increase the brightness information on the shadow region. In other words, the shadow region becomes brighter uniformly to an excessive level.
In a video camera, the value of the quantity of light in the backlight portion is always near "0". In an image data input from a color photographic film through a scanner, the value of the light quantity in the backlight portion does not always represent the pixel value near "0". For a photography printed with the backlight portion slightly brighter, the backlight portion is in the vicinity of, for example, the pixel value "30". With reference to FIG. 16, an input backlight unit 1604 corresponds to the backlight portion. Through the .gamma. correction for the image data, the brightness value of the backlight portion is converted into an output backlight portion 1605. As illustrated in FIG. 16, the range of the output backlight portion 1605 in this case is narrower than the range of the input backlight portion 1604. In other words, although the backlight portion in the image becomes brighter, the range of the brightness becomes narrower, thereby deteriorating the contrast condition and consequently the image quality.
The conventional backlight correction technique for digital image data, although it simulates the diaphragm control by a camera through the .gamma. correction for the data, is actually incapable of performing a good backlight correction in the correction of the digital data.